This invention relates to an optical recording medium capable of recording and reproducing information with laser beams, more specifically to an optical disc having a substrate provided with a read-only (ROM) area in which pre-pits are formed and a recordable (RAM) area in which pre-grooves are formed.
The compact disc (CD) has become the standard for high-speed, high-capacity Read Only Memory (ROM). In addition, there are also recordable (CD-R) and rewritable (CD-RW) CD technologies available. Advances in CD technology have increased both the speed of data transfer and the amount of data which a single CD can hold. Progress has been made in data transfer speed by spinning the disc faster during read and by more densely packing data in a two-dimensional space. Packing data more densely has also resulted in increased data storage capacity.
A substrate having a tope surface provided with concentric or spiral pre-grooves (guide grooves) and pre-pits may be prepared by a method including a series of steps of subjecting a photoresist layer formed on a glass to light exposure (laser cutting), developing and etching the resist layer, depositing and plating Ni to obtain a stamper, and then duplicating an information pattern for an optical disc from the stamper to a transparent plastic substrate by using a UV curable resin to prepare a replicated transparent substrate of an optical disc.
To obtain good tracking servo signal characteristics, the depth of guide grooves is usually set to be around xcex/8 to xcex/4 (wherein xcex is a wavelength of reproducing light). In contrast, because information such as track addresses, sector addresses or the like are formed into a pit-shape on the disc, the depth of each pit is set to be around xcex/4 to xcex/2. In this way, the depth of guide grooves is different from the depth of the pits.
An optical recording medium having a substrate provided with a read-only (ROM) area in which pre-pits are formed and a recordable (RAM) area in which pre-grooves, the depth of pre-grooves in the RAM area is different from the depth of the pre-pits in the ROM area. Further, the spacer portion between each adjacent pre-pits of the ROM area which portion is flush with top surface of the substrate differs in height from the pre-grooves in the RAM area. The known optical disc has a problem, because recording and reproducing cannot be performed in a stable manner especially when designed as a dense and large capacity optical disc.
It is an object of the present invention to provide an optical disc which has a minimized variation in tracking signals between the ROM area and in the RAM area and capable of stably performing the recording and reproducing of the information even when the disc is designed as a dense and large capacity optical disc.
Another object of the present invention is to provide an optical disc capable of keeping the physical continuousness of the ROM and RAM areas.
In accomplishing the foregoing objects, the present invention provides an optical disc comprising a substrate having a top surface provided with a ROM area and a RAM area, and a recording layer provided on said substrate, wherein said ROM area is provided with pre-grooves each having a bottom surface, wherein said ROM area has pre-pits formed on the bottom surface of said pre-grooves thereof, and wherein said RAM area is provided with pre-grooves.